1. Field of the Invention
The present invention concerns an apparatus for the application of a coaxial insulation to a conductor or an inner tube for the purpose of electrical insulation.
2. Description of Related Art
U.S. Pat. No. 3,737,116 discloses a winding machine for high voltage capacitor bushings which makes it possible to produce wound bodies up to a height of about 10.0 m., which is the maximum working width of the winding machine. An optimal feeding occurs when the width of the web of insulation material corresponds to the height of the body, e.g., the bushing insulation wound with the insulating material. However, such situations are limited because the maximum widths of the insulating material is limited to about 5.0 m. Webs of the insulating material must therefore be wound, for example, by overlapping the web, which, however, makes it difficult, even in the case of relatively small layer thicknesses, to produce such bodies without gaps. Gaps are formed by the partial separation of successive layers of the web, particularly due to the three-dimensional shrinkage reaction during the hardening of finished bushings impregnated with a plastic. The width of the gap may amount to several millimeters, but it may be as small as a microcrack. Gaps in high voltage insulation, for example, in high voltage bushings, are undesirable, as the air included in the gaps may be ionized, which may easily lead to punctures. Such insulation is not resistant to voltage surges.
It is possible to cut the incoming paper web during the winding, so that the wound body will assume a predetermined shape, for example, a conical form at both ends. The cuts parts are removed as waste.
It has therefore been proposed, for example, according to DE-OS No. 27 57 56, to apply two or more overlapping webs of insulating material to the winding body, and to cut, by a cutting device, the overlapping parts of the two adjacent webs of the insulating material prior to their application on the wound body. Between the two adjacent webs the smallest possible partition gap is created following the removal of the overlapping parts, and the winding and cutting devices are shifted axially if the former body is stationary. If, on the other hand, the cutting and winding devices are stationary, the former body is shifted axially so that the parting lines are covered from layer to layer, or after several layers, by the subsequent winding layer.
A disadvantage is the involved mode of operation and the relatively high waste of insulating material.
A further disadvantage is that, in view of the cutting means required between adjacent webs of the insulating material, parting gaps cannot be avoided.
A general disadvantage also consists of the fact that in the form of wound bodies provided with conducting inserts from webs of the insulating material, it is not possible to apply conducting inserts in a coaxially closed manner around a predetermined circumference of the wound body, as each insert is covered on both sides by the insulating material.
In order to produce a wound body free of gaps and microcracks, it has been proposed, according to DE No. 12 37 657, to use crepe paper without pressure in winding the layers. The crepe pattern of the crepe paper is retained, at least in part, during the winding process.
Although this results in improved insulating properties, at least in the direction of winding, i.e., in the diameter of the wound body, the insulating properties are not effected along the height axis, i.e., transverse to the direction of winding, because the elongation of the crepe paper is assured in the winding direction only, while an elongation of the crepe paper is desirable particularly in the height of the wound body, in view of the reaction shrinkage.
In all of the aforementioned processes for the production of wound bodies of an insulating material, the webs of the material are wound under a relatively high tensile stress, whereby undesirable tensile forces are applied to the webs of the insulating material and the risk of microcracking is increased. If crepe paper is used, most of the crepe pattern is lost in the winding process. In addition, the determination of the diameter of the body and thus the positioning of the inserts is rendered difficult.